Various applications of light guides are known in the art. A light guide operates on the principle of total internal reflection at the surface interface between the light guide material and the ambient medium. Light can be extracted from the light guide by altering the interface, such as by replacing the ambient medium with another material to frustrate the total internal reflection at that interface. Light guides have been useful for the transmission of optical information and for backlights.
For example, an illuminated image can be created by using a marker to deposit an illuminable ink on an active light guide. When ink, containing a light diffusing agent, is applied to the surface of this device, total internal reflection is “frustrated”, allowing light to be extracted into the ink, which then diffuses this light to produce an illuminated image. This effect can easily be produced by writing on an edge-illuminated light guide with a white crayon.
See-through graphic art image are also known in the art. One technique is now commonly used in advertisements, including printed graphics employed on automobile windows. In such see-through displays, a graphic image is printed on a perforated film or “perf-film”. These perforations allow a viewer of the back surface of the film to see-through the graphic image without seeing the graphic art displayed on the front surface of the film.